PCM transmission systems (PCM=Pulse Code Modulation), for instance telecommunication systems, are characterized in that information in a digital representation is transmitted in time slots. Each time slot transmits a predetermined number of bits, usually 8 bits. In conventional telephone systems these 8 bits are reserved for a channel. A predetermined number of such time slots (32 in Europe, 24 in the United States) are transmitted over a common PCM transmission link. In other words, a channel has access to a certain time slot, whereafter the channel is "silent" while the other channels have access to their respective slots. After the mentioned predetermined number of slots the channel once again has access to its time slot. Such a PCM transmission link, for instance a "PCM primary multiplex" in accordance with CCITT recommendation G732, G733, has a transfer rate of 64 k/bits (both in Europe and the United States).
In digital mobile telephony, speech information is coded by a speech coder in such a way that a lower transfer rate over the radio connection is obtained. In, for instance, the Group Special Mobile or GSM system, a transfer rate of less than 16 kbit/s, that is one fourth of the normal PCM transmission rate, is obtained. In this case, one channel occupies only two bits in a PCM word. In order to prevent the remaining bits from being unused, it has therefore been suggested that four such coded channels are packed into a PCM word that is sent to the public telephone network. This packing can for instance be done in a mobile services switching center.
In the future, there will also be so called "half rate" channels with a transfer rate of less than 8 kbit/s, which will require only one of the eight bits in a PCM word. In this case, it would also be desirable to pack several "half rate" channels in one and the same PCM word.
Furthermore, wirebound telephone systems are expected to use radio connections more and more also in the subscriber network itself, in which case the same code for speech will be used as in mobile telephony. In connection with a transfer from radio terminal to radio terminal in such systems and in cellular systems it would be desirable to maintain the compacter coding during the whole transfer path.
Data traffic with data rates less than 64 kbit/s have actually been switched in PCM networks by using the available words in a multiframe configuration (see CCITT recommendation V.100). However, multiframes introduce delays that may be tolerable for data but not for speech. For this reason, speech transmission with lower transfer rates than 64 kbit/s preferably should use one or two bits in a PCM channel rather than a word in a PCM multiframe.
Accordingly, there is a need for arbitrary switching of parts of a PCM word in such a way that the eight bits of the word are maximally utilized. This requires a bit oriented switch as a supplement to the word oriented switches of today.
U.S. Pat. No. 4,718,058 discloses a general switch capable of switching on both the word and bit level. However, this complicated general switch is supposed to replace existing switches for bit level switching capability. Thus, it is not intended to supplement existing switches with switching capabilities at bit level.